The present invention relates to a rotating stall prevention system for a compressor, and more particularly to a rotating stall prevention system which is suitable for preventing a rotating stall from taking place during increasing or decreasing speed of the compressor.
In a high pressure ratio axial compressor having a plurality of stator blade rows and a plurality of rotor blade rows arranged between the stator blade rows, there are some cases where a partial stalling region, referred to as a cell, is caused by flow separation from the blades, and this partial stalling region or cell rotationally propagates at a speed of approximately half the rotating speed of the compressor, resulting in a so-called rotating stall. The rotating stall phenomena is caused by the circumferential inlet distortion of the fluid flowing into the blade rows. Therefore, the rotating stall phenomena has been understood to include circumferential distortion of the fluid before it flows into the blade rows.
A system for preventing such a rotating stall is described in the ASME paper 91-GT-88 issued in Jul. 26, 1991. The system described in this paper has a plurality of hot wire anemometers arranged in the peripheral direction of a casing to detect a rotating stall (circumferential inlet distortion of flow), and the setting angles of a plurality of inlet guide vanes are controlled with mutual phase differences based on the detected signals so as to eliminate the rotating stall in the peripheral direction. The setting angles of the inlet guide vanes are changed with DC motors operated by a command from a control circuit.
The disadvantages in conventional rotating stall prevention systems may be eliminated by the above mentioned prior technology since an unsteady state flow field is directly controlled in the prior technology. However, varying the setting angles of a plurality of the inlet guide vanes with mutual phase differences may deteriorate the essential function of the inlet guide vanes. As a result, the inlet flow directions under a steady state flow condition into the blades in the rear stages differ depending on the peripheral positions thereof, which leads to serious effects on the performances under a normal operation of the compressor, such as an efficiency drop.